The Effect of Distance from Forest Edge on Seed Rain and Soil Seed Bank in a Tropical Pasture1

Abstract

Although lack of seeds can limit forest recovery in abandoned agricultural lands, few studies have documented the patterns of seed rain and soil seed bank in active pastures. To determine if lack of seeds is a limiting factor, we studied the woody species composition of the seed rain and soil seed bank in an active pasture in Barranquitas, Puerto Rico. In addition, fruit production of 43 common shrub and tree species was monitored in the surrounding secondary forest. Seed rain was monitored for one year at −2, 0, 2, 4, 8, 16, and 32 m from the forest edge along six transects. The soil seed bank was sampled in January/February 1995 and October/November 1995 at the same distances from the forest edge along ten transects. Of the 35 species that produced fruits in the forest, 14 species were detected in the seed rain study and only 0.3 percent of the seeds and 3 species dispersed to more than 4 m from the forest edge. Two of the three species were dispersed by wind. The seed bank study showed a similar pattern with a dramatic decrease in seedling density and species richness with distance from the forest edge. More than 50 percent of the seedlings in the seed bank study were Trema lamarckianum. This study demonstrated that few seeds disperse into the pastures and even when a rare dispersal event occurs, species do not accumulate because of short-term seed viability and, possibly, high seed predation. Studies of early secondary forest regeneration have shown that in some cases, forest can recover rapidly in abandoned agricultural lands; but our results suggest that only a small subset of the forest species will contribute to the initial recovery process.